1. Field of the Invention
The present invention relates to a silencer for an internal combustion engine suitable for application to a motorcycle or a four-wheeled motor vehicle or the like.
2. Description of Background Art
In an internal combustion engine used in a motorcycle or the like, if an exhaust noise is released as it is into the atmosphere, there occurs an explosive noise. To suppress the generation of such an explosive noise, a silencer is attached to the vehicle for allowing the exhaust gas to pass therethrough and absorbing sound waves to turn down the exhaust noise.
FIG. 6 schematically illustrates a longitudinal sectional configuration of a multi-stage expansion chamber type silencer 1. The silencer 1 has a generally cylindrical silencer body 2. The silencer body 2 comprises a circumferential wall 3, front and rear walls (front and rear partition walls) 4a, 4d which close the circumferential wall 3, and partition walls 4b and 4c as intermediate walls. With these walls there are formed first, second and third expansion chambers (first to third chambers) 5a, 5b, 5c.
An exhaust pipe 6 for introducing a gas (also called exhaust gas) G into the silencer body 2 is installed through the front partition wall 4a of the silencer body 2, while a tail pipe 9 for discharging the gas G from the interior of the silencer body 2 is installed through the rear partition wall 4d of the silencer body. Further, inner pipes 7 and 8 are installed through the intermediate partition walls 4b and 4c. In FIG. 6, arrows indicate flowing directions of the gas G.
In the silencer 1 shown in FIG. 6, the first to third expansion chambers 5a to 5c are contiguous to one another in three stages, but the silencer 1 can be expressed by way of a single chamber (single expansion chamber) silencer 12 as a model as illustrated in FIG 7.
As illustrated in FIG 7, the silencer 12 has a generally cylindrical expansion chamber 14 as a silencer body. The expansion chamber 14 is made up of a circumferential wall 16 having an overall length of L and front and rear partition walls 18, 20 which close the circumferential wall 16 and which has a diameter of .0.C.
An exhaust pipe 22 with a diameter .0.A for introducing the gas G into the expansion chamber 14 is installed through the front partition wall 18 of the expansion chamber 14, while a tail pipe 24 with a diameter of .0.B for discharging the gas G from the interior of the expansion chamber 14 is installed through the rear partition wall 20 of the expansion chamber. Also in FIG. 7, arrows indicate a flowing direction of the gas G.
A comparison will now be made between the first expansion chamber 5a in the silencer 1 illustrated in FIG. 6 and the silencer 12 as a single chamber model illustrated in FIG. 7. It is seen that the front partition wall 4a in the silencer 1 and the front partition wall 18 in the silencer 12 correspond to each other, that the partition wall 4b in the silencer 1 and the rear partition wall 20 in the silencer 12 correspond to each other, that the exhaust pipe 6 in the silencer 1 and the exhaust pipe 22 in the silencer 12 correspond to each other, and that the inner pipe 7 in the silencer 1 and the tail pipe 24 in the silencer 12 correspond to each other. Also as to the remaining second and third expansion chambers 5b, 5c in the silencer 1, they can be expressed likewise by the silencer 12 as a single chamber model illustrated in FIG. 7.
In the silencer 12 of FIG. 7, a noise deadening characteristic, an engine output characteristic (exhaust blow-through characteristic), and compactness are considered to be important points. It is known that the noise deadening characteristic is improved by enlarging the diameter .0.A of the exhaust pipe 22 and by shortening the distance, a, from a gas outlet end of the pipe 22 to an inner wall 26 of the rear partition wall 20 and the distance, b, from a gas inlet side of the tail pipe 24 to an inner wall 28 of the front partition wall 18 to increase the flow resistance of the exhaust gas G.
However, in the case where the flow resistance of the exhaust gas G is increased by shortening the distance, from the gas outlet end of the exhaust pipe 22 to the inner wall 26 of the rear partition wall 20 or the distance, b, from the gas inlet end of the tail pipe 24 to the inner wall 28 of the front partition wall 18, there arises the problem that the engine output becomes lower. In other words, there exists a reciprocal relation, so-called trade-off relation, for the noise deadening characteristic such that the exhaust blow-through characteristic of the engine is deteriorated.
In view of the above point and for making a desired noise deadening characteristic and a desired engine output characteristic compatible with each other, the applicant in the present case has experientially designed a silencer so that the ratio of the distance, a, to the diameter .0.A of the exhaust pipe 22 and the ratio of the distance, b, to the diameter B of the tail pipe 24 are (a/.0.A).gtoreq.1.2 and (b/.0.B).gtoreq.1.2, respectively.
The length of the exhaust pipe for communication of the expansion chamber (the expansion chamber 5a in FIG. 6) located on the most upstream side with an exhaust port of the internal combustion engine (not shown) is related to a torque characteristic relative to the rotational speed of the engine, while the diameter of the tail pipe which is open to the atmosphere from the expansion chamber (the expansion chamber 5c in FIG. 6) located on the most downstream side is related to the displacement of the internal combustion engine and the size of a normal rotational speed range.
Therefore, for attaining optimization while taking balance among the three characteristics of noise deadening characteristic, engine output characteristic and compactness, it has so far been required to adjust the distances a and b in each expansion chamber, adjust the overall length L and adjust the diameter of each inner pipe for connection between adjacent expansion chambers.